More precisely, the invention relates to a motor vehicle headlight of the type comprising a single light source, a reflector including two zones suitable for generating two different respective light beams, a closure glass, and masking means situated in the vicinity of the source and capable of being displaced for selectively masking the light rays delivered by the source and propagating towards at least one of the two zones.
A headlight of this type is already known in the prior art.
More precisely, French patent number 1 296 036 describes (page 2, righthand column) a particular embodiment of a headlight in which the filament is fixed relative to the reflector, and the reflector comprises a parabolic top portion whose focus is offset behind the filament in order to form a dipped beam, and a bottom portion which is also parabolic but which has its focus in the vicinity of the filament, so as to form a main beam in co-operation with the bottom portion. The two portions of the reflector are separated from each other at a horizontal plane including the optical axis, while the masking screen is suitable for selectively intercepting rays from the source propagating towards the bottom portion in order to change over between main beam and dipped beam.
A major drawback of a headlight of this type lies in the fact that since the dipped beam cut-off is directly created by the edge of the, or each, masking screen, and since mechanical play is inevitable in the design of such a moving masking screen, the position of the cut-off is not accurately defined, and this is not compatible with the photometric requirements laid down by various regulations.
This phenomenon is further accentuated in headlights of this type which satisfy current design trends, i.e. having a very small vertical extent while being very wide. This means that the cut-off is defined, at least with respect to the concentration images, by regions of the reflector which are very far from the source and the screen, thereby giving rise to undesirable amplification of any error in the positioning of the masking screen.
Another drawback of known headlights lies in the difficulty in enhancing the intensity of one beam to the detriment of the other. More precisely, given the abovementioned requirement for small vertical extent, any attempt at increasing the light in one of the beams requires the reflector to be made wider. Unfortunately, in this case, the other beam also benefits from such an increase, such that the ratio between the quantities of light specific to each of the two beams remains substantially the same.
In addition, French patent number 690 678 describes a headlight comprising a parabolic mirror having a matt or dull side zone which is permanently exposed to radiation from the source and having another zone which may be masked by a side masking element. This prior headlight is thus not capable of generating two different beams since it is only the maskable zone which is capable of forming a beam. This document is thus of no use in attempting to solve the problem to which the present invention relates.
The present invention seeks to mitigate the drawbacks of the prior art and to provide a headlight in which the photometric characteristics of the beam, and in particular the definition and the position of the cut-off of a dipped beam or of a foglight beam are independent of any possible play or inaccuracy that may exist in the position of a masking screen used for forming the beam in question.
Another object of the present invention is to provide a headlight of the type mentioned in the introduction and capable without difficulty of being very small in vertical extent, as required by present-day designs.
Yet another object of the present invention is to provide a headlight in which it is easy to modify the area of the reflecting zone attributed to each type of beam without having to modify the general configuration of the headlight.
Finally, the invention seeks to provide such a headlight in which the two beams formed can equally well be complementary or independent.